This invention relates to pulse signal handling arrangements and more particularly to circuit arrangements for recognising two or more pulses in a series by their mutual separations and indicating the time of arrival of one of the pulses in the series with respect to a known reference time. Pulse signal handling circuits such as this are commonly required for use in interrogator/transponder systems in, for example, airborne distance measuring equipment (DME).
This invention relates to a circuit for use in a transponder system for identifying response signals by their mutual separation in time and measuring the time of arrival of one of the pulses in relation to the time of transmission of an interrogation pulse. Such circuits are used in transponder systems for the purpose of airborne distance measurement, the TACAN system being an example of such an application.
In an airborne distance measuring equipment (DME) a transponder (transmitter/responder) aboard an aircraft transmits towards one of a set of navigation beacons on the ground a pair of pulses which are treated by the beacon as an instruction to send in reply two pulses of predetermined separation (after a predetermined time delay required for the purpose of processing). Upon receiving the response signals, the transponder aboard the aircraft first attempts to recognise the true response signals from spurious background signals by the mutual separation of the pulses, and measures the time of arrival of one of the received pulses in relation to the time that the interrogation was transmitted. The timing of arrival of the response is transmitted to a computer which, by subtracting the known processing delays inherent in the system arrives at a time representative of the travel time of radio waves to and from the beacon and by this method the distance of the aircraft from the beacon is calculated.
It is well known to discriminate between pulses of a known separation and spurious pulses by applying all the response pulses to a shift register having multiple tappings spaced apart by a number of stages corresponding to a time separation equal to the separation of the received signals. When a coincidence gating circuit detects the simultaneous presence of a response signal in each of the tapped stages then it is known that the sequence in the shift register is a true reply.
Since the accuracy requirements for distinguishing between true responses and spurious signals are not particularly stringent it is possible to utilise for the purpose of discrimination relatively slow acting shift registers. However, if it is desired to time the arrival of the first pulse in the series with respect to the time that the interrogation pulse was transmitted the coarse time quantisation effected by a shift register would destroy the accurate timing required for the purpose of distance measurement.